A straight-through serial cable is the most common type of serial cable, used to connect a Data Terminal Equipment (DTE) device (for example, a computer) to a Data Communications Equipment (DCE) device (for example, a modem). The pinout of the serial connectors on both sides of the cable are exactly the same with a straight-through serial cable.
A null modem serial cable (frequently called a crossover cable) is used to connect two DTE devices together without the use of a DCE device in between. For this to happen, the Transmit (TXD) and Receive (RXD) pins on one of the serial connectors are flipped.
pinout for straight through serial cable
The RS232 protocol is a popular serial interface that is used to connect computers to peripheral devices such as modems. We will take a look at the serial port pinouts used to implement RS232 as well as some additional reference information concerning the protocol.
RS232C retains the features of RS232 but uses 25 pins rather than a 9 pin serial pinout. Whether a DB9 serial pinout or a 25 pin connection is used, only three of the pins are required to connect terminal devices.
RS232 manages communication flowing between the DTE and DCE using serial pinouts of either the DB9 or DB25 variety. These D-sub connectors can terminate with an RS232 female pinout or DB25 or DB9 male connector pins. Each pin in a 9 or 25 serial connector pinout has its own distinct function. You can also learn RS485 pinout.
In addition to defining electrical characteristics, RS232 specifies the signals used in serial cable pinouts and serial ports. Familiar items such as timing signals and ground are included in these specifications.
Using the RS232 protocol you can connect two computers that do not have modems by using a null modem cable. This highlights one of the original uses of the RS232 protocol, which was developed in order to let teletype machines communicate with each other through their modems.
The other type of RS-232 Cable is the Straight-through cable. It is a one to one connector, It transmits a pin of one device that is connected to the transmit pin of another device and the receiver pin of one device is connected to the receiver pin of another device.
Null modem is a communication method to directly connect two DTEs (computer, terminal, printer, etc.) using an RS-232 serial cable. The name stems from the historical use of RS-232 cables to connect two teleprinter devices or two modems in order to communicate with one another; null modem communication refers to using a crossed-over RS-232 cable to connect the teleprinters directly to one another without the modems. It is also used to serially connect a computer to a printer, since both are DTE, and is known as a Printer Cable.
A null modem cable is a RS-232 serial cable where the transmit and receive lines are crosslinked. In some cables there are also handshake lines crosslinked. In many situations a straight-through serial cable is used, together with a null modem adapter. The adapter contains the necessary crosslinks between the signals.[2][3]
The popularity and availability of faster information exchange systems such as Ethernet made the use of null modem cables less common. In modern systems, such a cable can still be useful for kernel mode development, since it allows the user to remotely debug a kernel with a minimum of device drivers and code (a serial driver mainly consists of two FIFO buffers and an interrupt service routine). KGDB for Linux, ddb for BSD, and WinDbg or KD for Windows can be used to remotely debug systems, for example. This can also provide a serial console through which the in-kernel debugger can be dropped to in case of kernel panics, in which case the local monitor and keyboard may not be usable anymore (the GUI reserves those resources and dropping to the debugger in the case of a panic won't free them).
Another context where these cables can be useful is when administering "headless" devices providing a serial administration console (i.e. managed switches, rackmount server units, and various embedded systems). An example of embedded systems that widely use null modems for remote monitoring include RTUs, device controllers, and smart sensing devices. These devices tend to reside in close proximity and lend themselves to short run serial communication through protocols such as DNP3, Modbus, and other IEC variants. The Electric, Oil, Gas, and Water Utilities are slow to respond to newer networking technologies which may be due to large investments in capital equipment that has useful service life measured in decades. Serial ports and null modem cables are still widely used in these industries with Ethernet just slowly becoming a widely available option.
Connecting two DTE devices together requires a null modem that acts as a DCE between the devices by swapping the corresponding signals (TD-RD, DTR-DSR, and RTS-CTS). This can be done with a separate device and two cables, or using a cable wired to do this. If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal. One feature of the Yost standard is that a null modem cable is a "rollover cable" that just reverses pins 1 through 8 on one end to 8 through 1 on the other end.[1]
The simplest type of serial cable has no hardware handshaking. This cable has only the data and signal ground wires connected. All of the other pins have no connection. With this type of cable flow control has to be implemented in the software. The use of this cable is restricted to data-traffic only on its cross-connected Rx and Tx lines. This cable can also be used in devices that do not need or make use of modem control signals.[1]
A virtual null modem is a communication method to connect two computer applications directly using a virtual serial port. Unlike a null modem cable, a virtual null modem is a software solution which emulates a hardware null modem within the computer.[6][7] All features of a hardware null modem are available in a virtual null modem as well. There are some advantages to this:
The RS232 connector was originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present. Only very few computers have been manufactured where both serial RS232 channels are implemented. Examples of this are the Sun SparcStation 10 and 20 models and the Dec Alpha Multia. Also on a number of Telebit modem models the secondary channel is present. It can be used to query the modem status while the modem is on-line and busy communicating. On personal computers, the smaller DB9 version is more commonly used today. The diagrams show the signals common to both connector types in black. The defined pins only present on the larger connector are shown in red. Note, that the protective ground is assigned to a pin at the large connector where the connector outside is used for that purpose with the DB9 connector version.
The original pinout for RS232 was developed for a 25 pins sub D connector. Since the introduction of the smaller serial port on the IBM-AT, 9 pins RS232 connectors are commonly used. In mixed applications, a 9 to 25 pins converter can be used to connect connectors of different sizes. As most of the computers are equipped with the DB9 serial port version, all wiring examples on this website will use that connector as a default. If you want to use the example with a DB25, simply replace the pin numbers of the connector according to the conversion table below.
For the ProxySG, Advanced Secure Gateway, Content Analysis, Reporter, and Management Center hardware devices, a null model (crossover serial cable) is required for serial console access. For the most part the cable should have been included with the packaging of the device when it was shipped. However, there are times when this cable is misplaced for whatever reason and serial console access is required.
To use the computer's built-in serial port to communicate with a serial device, and it's difficult to determine if the serial cable is correct, see the solution section below to tell the difference between the two cable types of cables.
A null modem cable (crossover cable) should be used when interfacing directly between the computers serial port and the serial port on the Symantec products mentioned above. With null modem cables, the transmitting device's Transmit (TX) signal is sent to the receiving device's Receive (RX) line. Similarly, the receiving device's transmission line is sent to the communicating device's receiving line.
Rollover wired cables, most commonly called rollover cables, have opposite Pin assignments on each end of the cable or, in other words, it is "rolled over." Pin 1 of connector A would be connected to Pin 8 of connector B. Pin 2 of connector A would be connected to Pin 7 of connector B and so on. Rollover cables, sometimes referred to as Yost cables are most commonly used to connect to a device's console port to make programming changes to the device. Unlike crossover and straight-wired cables, rollover cables are not intended to carry data but instead create an interface with the device.
We received an email recently pointing out some issues with NULL modem cables. The pinouts shown below will generally work. However, there are many permutations of signal sets that can be used by either end of a connection and they may not be SYMMETRIC. One end may expect something (a signal) that the other end cannot generate. This typically happens with CTS/RTS (and perhaps DCD) and DTR/DSR. If you suspect this is the case then unfortunately you need to understand the interface and may have to 'spoof' (artifically create) certain signals. Our signal primer page may help you. Finally, if you are having serious problems, splash out on what is frequently called a 'light box' or some other device that will show you which signals are being activated. 2ff7e9595c
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